Creating a custom Java process extending AbstractAlgorithm

This tutorial is very similar to Extend ing AbstractSelfDescribingAlgorithm. However, if you extend AbstractAlgorithm you are able to create the ProcessDescription file yourself.

Download the process development kit or the WPS extension skeleton project.
  • Create a new package: org.n52.wps.demo
  • Create a new class: AbstractAlgorithmExample
  • Let the new class extend AbstractAlgorithm

Implement Process

Add the following fields to the class:

private final String inputID = "input";
private final String outputID = "output";

For AbstractSelfDescribingAlgorithm, you need to implement four methods:

Method 1: public Class getInputDataType(String id)

This method lists all input datatypes for a given input data identifier.In our case, we only have one input identifier: inputID. The return class must be a specific binding.

We will simply use a LiteralStringBinding here:

    return LiteralStringBinding.class;
return null;

Method 2: public Class getOutputDataType(String id)

This method lists all output datatypes for a given output data identifier.In our case, we only have only one output identifier: outputID. We will use the LiteralStringBinding again here:
    return LiteralStringBinding.class;
return null;

Method 3: public List<String> getErrors()

This method can be used to pass error descriptions to clients. For this example, we will not use it. Just keep the
 return null;


Method 4: public Map<String, IData> run(Map<String, List<IData>> inputData)

This method holds the business logic. At first we check that all necessary inputdata is present.

if (inputData == null || !inputData.containsKey(inputID)) {
        throw new RuntimeException("Error while allocating input parameters");
List<IData> dataList = inputData.get(inputID);
if (dataList == null || dataList.size() != 1) {
        throw new RuntimeException("Error while allocating input parameters");

Next, we get the string binding:
IData stringInputData = dataList.get(0);

We will just return the string input. So the last step is to create the standard output hashmap which holds the name of the output identifier and an IData object. In our case we just pass the string input binding:
HashMap<String, IData> result = new HashMap<String, IData>();
result.put(outputID, stringInputData);
return result;

Save the source code and compile it. You may need to "Organize imports" to add all required classes. You can download the complete Java class here.

Create the ProcessDescription

When our process is extending AbstractAlgorithm, the ProcessDescription is not generated automatically as with other algorithm base classes. So we have to create it ourselves. This gives us more control e.g. over the input and output formats our process should support. The following is a common header, where the namespaces and schemalocation is defined:
<?xml version="1.0" encoding="UTF-8"?>
<wps:ProcessDescriptions xmlns:wps="" xmlns:xsi="" 
xmlns:ows=""xsi:schemaLocation="" xml:lang="en-US" service="WPS" version="1.0.0">

Now we can define whether status messages and result storage are supported and also the process version:
<ProcessDescription statusSupported="true" storeSupported="true" wps:processVersion="1.1.0">

In the following section the inputs are defined:
 <Input minOccurs="1" maxOccurs="1">
 <ows:DataType ows:reference="xs:string"/>

Note that the identifier must be the same as inputID in the Java class. The process outputs are defined as follows:
 <ows:DataType ows:reference="xs:string"/>

In this case the identifier must be the same as outputID in the Java class. Now we just close the open tags:

Save the Processdescription as AbstractAlgorithmExample.xml. You can download the complete Java class here.

Deploy the custom process

See here.

Execute the process

See here.

Topic revision: r4 - 22 May 2015, DanielNuest
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